Losses and Loss Expenses - Reconciliation of the beginning and ending balances of unpaid losses and losee expenses (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss expenses, beginning of period		$ 29,696,779	$ 25,939,571
Unpaid losses and loss expenses recoverable, beginning of period		7,247,723
Net unpaid losses and loss expenses, beginning of period		22,457,333	20,459,271
Increase (decrease) in net losses and loss expenses incurred in respect of losses occurring in:
Current year		3,361,365	3,175,267
Prior year		(18,050)	(62,728)
Total net incurred losses and loss expenses		3,343,315	3,112,539
Foreign exchange and other		(107,166)	292,040
Less net losses and loss expenses paid in respect of losses occurring in:
Current year		277,656	343,934
Prior year		3,432,843	2,547,073
Total net paid losses		3,710,499	2,891,007
Net unpaid losses and loss expenses, end of period		21,982,983	20,972,843
Unpaid losses and loss expenses recoverable, end of period		7,209,084
Unpaid losses and loss expenses, end of period		29,181,147	26,822,346
Total P&C
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss expenses recoverable, beginning of period	[1]	7,239,446	5,480,300
Less net losses and loss expenses paid in respect of losses occurring in:
Unpaid losses and loss expenses recoverable, end of period	[1]	$ 7,198,164	$ 5,849,503

[1] P&C business only, net of provision for uncollectible reinsurance.